This invention relates to gas permeable fibrous bodies, more particularly for use as tobacco smoke filters in cigarettes and other smoking articles. It is anticipated, however, that the primary use for fibrous bodies in accordance with the invention will be in the field of cigarette filters. Accordingly, the discussion which follows will be oriented toward this application of the invention but without limiting intent.
Considerable effort has been expended in recent times in developing the ultimate cigarette filter. In fabricating filters for use in connection with cigarettes and the like, a number of considerations, frequently conflicting, must be taken into account and balanced. Filtration efficiency (i.e., the ability of a filter to remove undesirable constituents from tobacco smoke) is clearly a primary consideration, but this must frequently be compromised in order for the filter to possess other properties which make it commercially practicable and acceptable. Such other properties include pressure drop, taste, hardness and cost. Generally, it may be stated that a major object in the development of the optimum cigarette filter is to obtain maximum filtration efficiency consistent with an acceptable combination of the other parameters.
Reasonably high filtration efficiency, or so called "low tar" cigarette filters have been available for some time. An effective filter of this type is disclosed, for example, in my earlier U.S. Pat. No. 3,533,416. This filter found significant commercial success but is rather high in cost and has a limit of about 70% filtration. Relatively high filtration efficiencies have also been obtained with ventilated-type filters in which the smoke is diluted with inhaled air, but such filters tend to suffer from a loss of taste.
An alternative approach to the manufacture of high efficiency cigarette filters was a proposal disclosed, for example, in prior U.S. Pat. Nos. 3,595,245 and 3,615,995 which utilizes a tow or roving of extremely fine entangled melt blown plastic fibers to form a filter rod. In the melt blown manufacturing process, a large array of fibers are extruded in a stream of pressurized gas through individual extrusion dies in a die head, and the fibers are gathered together by one or another technique to form an entangled tow or roving ultimately used to form a filter rod. The melt blown process can be controlled to produce extremely fine fibers down to about 0.01 microns in diameter and accordingly the process would appear to lend itself to the production of filter rod material of very high filtration efficiency because of the large retention area available in a tow or roving made from such fine fiber material.
In practice, however, applicant is unaware of any cigarette filter made by the melt blown process which had a filtration efficiency substantially over 70%. This is believed to be due to the fact that to obtain a usable pressure drop with such ultra-fine fibers, the bulk density of the filter should be below about 0.1 gram per cc, and with ultra-fine fibers below about 8 microns in diameter, if the bulk density is reduced below 0.1 gram per cc, the physical properties of the resultant product, particularly its structural integrity, are unacceptable for use as a cigarette filter. While resins and other bonding agents can be added to the tow to improve its structural integrity, these can affect both pressure drop and filtration characteristics.